In most of these cases I don't think the VMware administrators are setting these memory limits to be equal to the assigned memory. I also see this at many places and when I show it to the VMware administrators often they did not even know that they could set a memory limit.
I think there was an issue with an older version of vCenter, probably back in the v3.5 days, that set these limits and they are still set because the administrators are unaware of them. PowerGUI with the vmware admin pak has a script to check for memory limits on VMs and removing them. I normally use this at new customers I visit. If the limits are still set at the same as the assigned memory it is not an issue, it is when the assigned memory has been increased on these VMs and the limit left in place because the administrator did not know about it, this starts to cause performance issues with the administrators wondering why a VM that they have added more memory to is now performing worse than it was before the memory was increased.